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Abstract:

A method, which allows extremely high-tensile flat steel products to be
manufactured with less effort includes a steel that forms a martensitic
microstructure and contains (in wt. %) C: 0.15-0.19%, Mn: 0.80-1.20%, P:
<0.030%, S: <0.004%, Si: 0.60-1.00%, Al: <0.05%, N: <0.0060%,
Cr: 0.30-0.60%, Nb: 0.040-0.070%, remainder iron and unavoidable
impurities, being cast into a cast strip having a thickness of 1-4 mm.
The cast strip is hot-rolled in-line into a hot-rolled strip having a
thickness of 0.5-3.2 mm in a continuous process at a final hot-rolling
temperature ranging from 900 to 1050 ° C., the deformation degree
being greater than 20%. The hot-rolled strip is coiled at a coiling
temperature of at most 350° C., so as to obtain a hot-rolled
strip, which has a minimum tensile strength Rm of 1400 MPa at a
minimum breaking elongation A80 of 5%.

Claims:

1. Method for manufacturing flat steel products,wherein a steel that forms
a martensitic microstructure with the following composition (in wt. %)C:
0.15-0.19%Mn: 0.80-1.20%P: ≦0.030%S: ≦0.004%Si:
0.60-1.00%Al: ≦0.05%N: ≦0.0060%Cr: 0.30-0.60%Nb:
0.040-0.070%remainder iron and unavoidable impurities is cast into a cast
strip having a thickness of 1-4 mm,wherein the cast strip is hot-rolled
in-line into a hot-rolled strip having a thickness ranging from 0.5 to
3.2 mm in a continuous process at a final hot-rolling temperature ranging
from 900 to 1050.degree. C., the deformation degree being greater than
20% andwherein the hot-rolled strip is coiled at a coiling temperature of
at most 350.degree. C.,so as to obtain a hot-rolled strip, which has a
minimum tensile strength Rm of 1400 MPa at a minimum breaking
elongation A80 of 5%.

2. Method according to claim 1, wherein the width of the hot-rolled strip
is greater than 1,200 mm.

3. Method according to claim 1, wherein the thickness of the hot-rolled
strip is 1.5 mm at most.

4. Method according to claim 1, wherein the hot-rolled strip is
cold-rolled into cold-rolled strip having a thickness of 0.5-1.4 mm.

5. Method according to claim 4, wherein the cold-rolled strip is annealed
at an annealing temperature of 750-850.degree. C.

6. Method according to claim 4, wherein a minimum tensile strength of the
cold-rolled strip is 600 MPa.

7. Method according to claim 4, wherein the cold-rolled strip has a
minimum breaking elongation A50 of 15%.

8. Method according to claim 1, wherein the hot-rolled strip or cold
rolled strip is provided with a metallic coating.

9. Method according to claim 8, wherein the metallic coating is a zinc
coating.

10. Method according to claim 4, wherein the cold-rolled strip is provided
with a metallic coating.

11. Method according to claim 10, wherein the metallic coating is a zinc
coating.

Description:

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001]This application is a National Phase Application of International
Application No. PCT/EP2007/061389, filed on Oct. 24, 2007, which claims
the benefit of and priority to European patent application no. EP 06 123
137.9, filed on Oct. 30, 2006. The disclosures of the above applications
are incorporated herein by reference in their entirety.

FIELD OF THE INVENTION

[0002]The invention relates to a method for manufacturing flat steel
products, such as strips or sheet metal blanks, from high-tensile
martensitic steels. Such MS steels belong to the group of multi-phase
steels. These usually concern steels, the properties of which are
determined by type, quantity and alignment of the phases of the
microstructure. Therefore at least two phases exist in the microstructure
(ferrite, martensite, bainite for example). As a result they have a
superior strength/formability combination compared to conventional
steels.

BACKGROUND

[0003]Because of these special features multi-phase steels are of major
interest particularly for automotive construction, since due to their
high strength on the one hand they allow the use of smaller material
thicknesses and consequently at the same time a reduction in the vehicle
weight and on the other hand improve the safety of the vehicle body in
the event of a collision (crash behavior). Thus multi-phase steels with
at least equal strength of the overall body permit a reduction in the
sheet metal thickness of a component made from such multi-phase steels
compared to a body made from conventional steels.

[0004]Usually, multi-phase steels are melted in a converter steel mill and
cast on a continuous casting machine into slabs or thin slabs, which are
then hot-rolled into hot-rolled strip and coiled. In this case the
mechanical properties of the hot-rolled strip can be varied by
selectively controlled cooling of the hot-rolled strip after hot-rolling
with the aim of adjusting certain microstructural fractions. The
hot-rolled strip can also be cold-rolled into cold-rolled strip in order
to also obtain thinner sheet metal thicknesses (EP 0 910 675 B1, EP 0 966
547 B1, EP 1 169 486 B1, EP 1 319 725 B1, EP 1 398 390 A1).

[0005]A problem with manufacturing flat products from high-tensile
multi-phase steels with tensile strengths of more than 800 MPa is that
high rolling forces must be applied when rolling such steels. This
requirement has the consequence that normally with the production
machines at present generally available, high-tensile hot-rolled strip
made from steel of the type under discussion can often only be
manufactured in a width and thickness, which no longer fully meet the
requirements demanded today by the automotive industry. In particular,
strip of narrow thickness with sufficient width cannot be produced very
well in conventional installations. Also, with conventional methods it is
shown in practice that it is difficult to manufacture cold-rolled strip
with a strength of more than 800 MPa from multi-phase steel.

[0006]An alternative way to produce steel strip from a multi-phase steel
has been proposed in the European Patent EP 1 072 689 B1 (DE 600 09 611
T2). In accordance with this known method to produce thin steel strip
firstly a steel melt, which (in wt. %) contains 0.05 and 0.25% C, in
total 0.5-3% Mn, Cu and Ni, in total 0.1-4% Si and Al, in total up to
0.1% P, Sn, As and Sb, in total less than 0.3% Ti, Nb, V, Zr and REM
(i.e., rare earth metals) as well as in each case less than 1% Cr, Mo and
V, remainder iron and unavoidable impurities, is cast into cast strip
having a thickness of 0.5-10 mm, in particular 1-5 mm. The cast strip is
subsequently hot-rolled in-line into a hot-rolled strip in one or more
passes, the deformation degree ranging between 25% and 70%. The final
hot-rolling temperature in this case is above the Ar3 temperature.
At the end of hot-rolling the obtained hot-rolled strip is then cooled
down in two steps. In the first step of this cooling a cooling rate of
5-100 ° C. per second is maintained until a temperature ranging
between 400-550 ° C. is reached. The hot-rolled strip is then held
at this temperature for a dwell time, which is needed in order to allow
bainitic transformation of the steel with a residual austenite content
greater than 5%. The formation of pearlite in this case is to be avoided.
After a dwell time sufficient to obtain the required microstructure, the
transformation process is interrupted by the beginning of the second
cooling step, wherein the hot-rolled strip is brought to a temperature
below 400° C., in order then to wind it into a coil at a coiling
temperature below 350° C.

[0007]With the method described in EP 1 072 689 B1, it should be possible
to produce hot-rolled strip with bainitic microstructural fractions in a
simple way from a multi-phase steel, which has TRIP characteristics
("TRIP"="Transformation Induced Plasticity"). Such steel has relatively
high strength with good formability. However, the strength is not
sufficient for many applications, particularly in the field of automotive
construction.

SUMMARY OF THE INVENTION

[0008]In general, an aspect of the invention is to provide a method, which
allows extremely high-tensile flat steel products to be manufactured with
less effort in a wide range of geometrical dimensions.

[0009]In one embodiment, the aspect indicated above has been achieved by a
method for manufacturing flat steel products, wherein according to the
invention a steel that forms a martensitic microstructure, which (in wt.
%) contains 0.15-0.19% C, 0.80-1.20% Mn, up to 0.030% P, up to 0.004% S,
0.60-1.00% Si, up to 0.05% Al, up to 0.0060% N, 0.30-0.60% Cr,
0.040-0.070% Nb and remainder iron and unavoidable impurities, is cast
into a cast strip having a thickness of 1-4 mm, wherein the cast strip is
hot-rolled in-line into a hot-rolled strip having a thickness of 0.5 to
3.2 mm in a continuous process at a final hot-rolling temperature ranging
from 900 to 1050° C., the deformation degree being greater than
20%, and wherein the hot-rolled strip is coiled at a coiling temperature
of at most 350° C., so as to obtain a hot-rolled strip, which has
a minimum tensile strength Rm of 1400 MPa at a minimum breaking
elongation A80 of 5%.

[0010]The invention provides a method of casting to convert a particularly
high-tensile, martensitic steel into a hot-rolled strip. Since the cast
strip itself in this case already possesses a narrow thickness, only
relatively low deformation degrees must be maintained in the course of
hot-rolling this strip, in order to manufacture flat products with narrow
thicknesses, as they are required particularly in the field of automotive
construction. Thus it is possible with the method according to the
invention, by specifying a corresponding initial thickness of the cast
strip, to produce without any problems hot-rolled strip, which with an
optimal characteristic distribution has a maximum thickness of 1.5 mm and
from which components for the support structure of a vehicle for example
can be manufactured.

[0011]Due to the low deformation degrees during hot-rolling, the rolling
forces necessary for this, compared to the forces necessary for
hot-rolling slabs or thin slabs with the conventional method, are low, so
that hot-rolled strip of large width, which lies substantially above the
width of hot-rolled strip of the same strength and thickness cast in the
conventional way, can be produced without any problems with the method
according to the invention. Thus, the invention permits high-tensile
hot-rolled strip, consisting of a martensitic steel with the composition
indicated and processed according to the invention, the width of which is
greater than 1,200 mm, in particular greater than 1,600 mm, to be
reliably produced.

[0012]The application according to the invention of the strip casting
process for converting high-tensile steels of the type composed according
to the invention, apart from the advantages mentioned above, due to their
characteristics and process variables specific to the method (hot-rolling
final temperature, cooling, coiling temperature for example) offers the
possibility, also in respect to their solidification behavior, of
reliably casting critical steel compositions of the type processed
according to the invention. Thus very rapid solidification of the cast
strip, characteristic for strip casting, leads to a substantially reduced
risk of the emergence of center liquations, compared to conventional
production, with the consequence that the hot-rolled strip produced
according to the invention has a particularly uniform characteristic
distribution and microstructure over its cross section and its length.

[0013]A further special advantage of the method according to the invention
is that the hot-rolled strip produced according to the invention has a
high strength of at least 1400 MPa, without in addition a special cooling
cycle of the hot-rolled strip having to be maintained between the end of
hot-rolling and coiling, which is prescribed for example in EP 1 072 689
B1 as the result of the need for a cooling interruption. In carrying out
the method according to the invention, it must only be ensured that
hot-rolling is terminated in a relatively closely confined temperature
window and also that coiling is carried out in a precisely defined
temperature range. Single-step cooling takes place in the interim.

[0014]A further advantage of the method according to the invention is that
an extension in the range of mechanical properties of the strip produced
according to the invention can be achieved, based on a single steel
analysis, by varying the cooling and rolling conditions.

[0015]Hot-rolled strip produced according to the invention is particularly
suitable for subsequent conversion into cold-rolled strip. Accordingly,
one practical embodiment of the invention makes provision for the
hot-rolled strip to be cold-rolled into cold-rolled strip having a
thickness of 0.5-1.4 mm, in particular 0.7 mm up to 1.3 mm, as is needed
for constructing automotive bodies. In order to eliminate solidifications
arising during cold-rolling, the cold-rolled strip can be annealed at an
annealing temperature of 750-850° C. For cold strip produced in
this way from hot-rolled strip manufactured according to the invention, a
minimum tensile strength of 800 MPa can be reliably ensured. At the same
time just as reliably the minimum breaking elongation A50 of the
cold-rolled strip is 10%.

[0016]In accordance with a further advantageous embodiment of the
invention the cold-rolled strip is provided in the way known per se with
a metallic coating, in which, for example, this can be a zinc coating.

DESCRIPTION

[0017]The invention is described in detail below on the basis of exemplary
embodiments. In trials carried out to demonstrate the effect of the
invention, two steels A and B composed according to the invention with
the composition indicated in Table 1 are melted and, in a conventional
two-roll casting machine, each cast into cast strip, which was 1.6 mm
thick.

[0018]The strips cast from the steels A and B have been hot-rolled in-line
directly after the strip was cast into a hot-rolled strip, having a
thickness of 1.25 mm, at a final hot-rolling temperature WET.
Subsequently, the obtained hot-rolled strip in each case was directly
cooled in a cooling step to a coiling temperature HT and coiled. After
coiling the hot-rolled strips produced from the steels A and B in each
case had a tensile strength Rm and a breaking elongation A80,
which are indicated in Table 2 as the final hot-rolling temperature WET
and coiling temperature HT maintained in each case during their
production.

[0019]The hot-rolled strip produced from the steel A, after coiling and
pickling, was cold-rolled into a 0.7 mm thick cold-rolled strip and
annealed in-line at a temperature of 780° C., in order to
recrystallize the strip.

[0020]With a breaking elongation A50 of 21.8%, the tensile strength
Rm of the cold-rolled strip obtained in this way was 654 M